1. Field of This Invention
This invention concerns equipment for continuous plating of prestamped or bandoliered metal parts, which are transported lengthwise in the equipment, through tanks filled with electrolytes.
2. Prior Art
The electronic industry uses many parts which, which require for proper functioning a coating of precious metals, such as gold silver or paladium.
The high prices of precious metals will urge manufacturers to use these metals as economically as possible. This can be achieved because usually only a small area of the particular component requires a coating with precious metal for proper functioning; the remainder can be coated with a cheaper metal, such as nickel or tin, or does not require a coating at all.
Examples of parts requiring treatment as indicated are stamped frames for semiconductors, e.g., transistors, integrated circuits etc., and contact parts for the connector industry. This partial coating by electrolysis is usually called functional or selective plating.
An obvious method for selective plating is application of metal coatings as spots or strips on flat strip metal before stamping. This method offers large effective savings because flat strip can be plated accurately to a specific pattern by using nonconductive coatings or points to mask off the areas which do not require a metal coating.
Alternatively mechanical masking can be used.
Euipment for this type of selective plating has been the subject, for example, of the following Patents: U.S. Pat. Nos. 6,038,169; 4,069,125; 4,072,581 and Dutch Pat. No. 7,107,171.
These methods also have disadvantages; an obvious one is that the sides of the final product after stamping are bare and hence can corrode. The corrosion products often spread over the functional area. Further, stamping may lead to damage of the metal coating, or may cause cracks in the precious metal deposit.
To avoid these disadvantages components can also be plated after stamping, provided some form of interconnection is left between the parts to make them form a continuous strip or ribbon. Alternatively, loose parts can be assembled in an auxilliary strip to obtain the necessary bandoliered system. Such laddered or bandoliered strips of components can be plated at high speed in suitable equipment with high speed. Examples of typical machines for this type of operation can be found in U.S. Pat. Nos. 4,029,555 and 4,032,414 and in German Pat. No. 2,636,413.
The disadvantages of these machines are their limited field of application. They are typically designed for production of large numbers of the same component, and require costly tooling and involve great loss of time to be made suitable for different components.
In many cases however it is desirable to change over from production of one component to the next one of different shape without elaborate work and or costs. One method to achieve this target is to use the system of partial immersion or controlled depth plating. The laddered or bandoliered components are transported vertical through an electrolyte, whereby only that part of the component requiring a precious metal coating is immersed in the electrolyte. In many cases this can be done because the local deposit is wanted on one end of the component. This is particularly useful for contact parts and certain semiconductor frames. Change from one type of product to another is then often only a matter of regulation of solution level.
Although this type of continuous selective plating seems attractive, it has some serious disadvantages: it is slow and inaccurate. The deposition speed of a plating system depends largely on the degree of solution agitation at the surface to be plated. Strong electrolyte agitation can be otained by jetting the electrolyte onto the surface to be plated or by introducing air with the solution. An electrolyte containing 10-12 g/l of gold metal will at 45.degree. C. deposit 1 micro-meter in 10 minutes without solution agitation. With light agitation this time can be reduced to 3 minutes, with electrolyte jetting to 10 sec.
When parts are submersed over a length of 1 to 10 mm into an electrolyte, this electrolyte cannot be strongly agitated without disturbing the solution level to such a degree that accurate immersion depth is lost. To obtain therefore an acceptable level of plating accuracy, agitation must be low and hence plating speed is slow. To for the plating speed, the length of the selective plating operation could be increased. This however leads to other problems. All stamped products show a certain degree of camber and require therefore accurate guiding to control straightness of the laddered components over a greater length. Such guiding implies a nonflexible complicated construction and is therefore a serious objection against using long plating sections for controlled depth plating.
As a solution for this problem, it has been proposed to use a transport mechanism to which component strips are connected during passage through the plating machine. Although this is a solution for some products, it is impossible to use it for such parts which require a double sided treatment, such as gold on one side and tin on the other side, which is a frequent specification.
Finally it is desirable to transport components which require selective plating with a continuous constant speed through an electrolyte to obtain maximum evenness of precious metal coatings. The use of intermittant indexing systems frequently leads to substantial differences in coating thickness from one part to the other due to difference in anode or agitation activities.